A nurse is caring for a client who receives furosemide to treat heart failure. Which of the following laboratory values should be a priority for the nurse monitor due to this medication?

Rationale: A. Magnesium is incorrect as the highest priority because, although furosemide can increase renal excretion of magnesium, hypomagnesemia is less immediately life-threatening compared with hypokalemia. Low magnesium can contribute to cardiac arrhythmias, but monitoring magnesium is generally secondary to monitoring potassium in acute management of heart failure. B. Calcium is incorrect because loop diuretics like furosemide can increase calcium excretion, potentially leading to hypocalcemia over time. However, calcium disturbances are usually gradual and less acutely dangerous than potassium imbalances, so they are not the priority concern in a client at risk for cardiac complications. C. Sodium is incorrect because furosemide can cause hyponatremia by promoting sodium loss in the urine. While significant sodium depletion can lead to neurological symptoms (confusion, seizures) and fluid imbalance, the risk of sudden, life-threatening cardiac arrhythmias is greater with potassium loss, making potassium monitoring more urgent. D. Potassium is correct because furosemide potently increases renal potassium excretion, which can result in hypokalemia. Potassium is essential for normal cardiac conduction, and deficiency can quickly lead to ventricular dysrhythmias, cardiac arrest, muscle weakness, and fatigue. Monitoring potassium, assessing ECG changes, and providing dietary supplementation or medication adjustments are critical to prevent these potentially fatal complications.